Many varieties of trunked two-way radio communications systems are known. FIG. 1 is a block diagram illustrating both a typical conventional radio system 101 and a trunked radio system 103. The communications systems of FIG. 1 may be digital or analogue.
In the conventional radio system 101, a plurality of mobile communication units SU1, SU2, . . . SU9 are formed into talkgroups A, B or C. Each talkgroup uses a separate channel CH1, CH2 or CH3 for communication. Thus, each talkgroup is served by one channel. Henceforth the term ‘mobile communication unit’ will be used for any wirelessly linked mobile communication unit that may be linked to the wireless communication systems discussed in the remainder of this description. The mobile communication unit may be a phone, mobile or portable radio, a smartphone, or another wirelessly linked mobile communication unit, among other possibilities.
The trunked radio system 103 and its mobile stations SU1, SU2, . . . SU9 use a pool of channels CH1, CH2 and CH3. These channels can support a virtually unlimited number of talkgroups. Thus, all talkgroups may in fact be served by any channel, and may well be served by all channels at different times. The trunked radio system 103 works to take advantage of the probability that not all talkgroups will need a channel for communication at the same time. Estimates are made about how much load a typical user will present to the system, in terms of calls per hour and duration of each call.
For a given traffic load, the trunked radio system 103 requires fewer channels, since all talkgroups can be served by all channels. With any given number of channels, the trunked radio system 103 can accommodate a much greater number of talkgroups than conventional radio systems, such as radio system 101. Hence, a primary benefit of a trunked radio system is the efficient utilization of channels. The trunked radio system allows more users to carry on more conversations, over fewer distinct channels. This applies to data and/or voice calls.
In both conventional and trunked communication systems, each call is assigned one channel. The channel comprises two frequencies. At a base station of each communication system, one frequency is used to receive a call from a mobile station. This first frequency is referred to as the receive frequency (Rx). The function of the base station is to re-transmit the call to the other members of a talk group. A second frequency is used for that re-transmission. This second frequency is referred to as the transmit frequency (Tx). In such communications systems, a console may be operated by a dispatcher, who can communicate with one or more subscribers. In multicast or simulcast systems, there may be a comparator to distribute signals to different base stations for transmission.
When a mobile communication unit initiates a new call in the known systems of FIG. 1, a receiver of the communication system receives the signal, if the signal is valid. The received signal is processed, and sent to the infrastructure of the communications system. For example, a console operator may receive the subscriber user voice/data from the mobile communication unit.
When the systems of FIG. 1 operate in ‘repeat’ mode, the signal received from a mobile communication unit is processed by a base station, and ‘repeated’ on the Tx frequency. ‘Repeat’ means that the received signal is re-transmitted, typically to multiple members of a talkgroup. This allows mobile communication units to communicate to one or more other mobile communication units, as well as to the console of the communications system. The repeat function may be controlled, i.e. enabled or disabled, by a console operator or a system administrator. Only a single call from a mobile communication unit can be handled by one base station at a time. So a second mobile communication unit cannot start a call when another call, from a first mobile communication unit is ongoing. Effectively the channel assigned to the base station channels is busy. Signals from two mobile communication units that tried to transmit at the same time would interfere. There would be no certainty that either signal would be recognized by base station as a valid signal. In many situations, neither the first nor the second communication unit would succeed in transmitting.
This situation may have adverse consequences when there is an uplink call ongoing from a first mobile communication unit to a base station, and a second mobile communication unit needs to place an emergency call. This might be when the user of the second mobile communication unit is in danger. The second mobile communication unit cannot initiate an emergency call, as it is not possible to preempt the ongoing uplink call, and typically the user of the second mobile communication unit simply has to wait. Thus a console operator will not receive a call from the second mobile communication unit, at a point in time when such a call really matters. A less urgent situation may occur when a second mobile communication unit needs to make a call of higher priority than a call that is already ongoing. Henceforth the term ‘preemption call’ will be used to describe an emergency call, or a call of higher priority than an ongoing call.
A conventional solution to this problem is to use an additional dedicated frequency to provide emergency call support. The second mobile communication unit will use the additional frequency for emergency calls. This solution ensures that a console operator will hear the emergency call, even when a call is already ongoing through the base station. However, obtaining an additional frequency is expensive. An exclusive emergency frequency would also be idle at most times.
Another known solution is to create a ‘hole’ in an audio stream, to allow for higher priority subscribers to interrupt. An interrupt message can be used to signal that a ‘higher priority’ second mobile communication unit would like to take over transmission from a first mobile communication unit that is already using a channel.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.